1. Technical Field
A multiplexed electrospray deposition apparatus capable of delivering picoliter volumes of one or more substances is disclosed. The apparatus may include a unitary planar dispenser etched from a silicon wafer through microfabrication or micromachining technology. The apparatus may be used as a deposition tool for making protein microarrays in a noncontact mode. Upon application of potential difference in the range of 7-9 kV, the substances may be dispensed directly, not through a collimating mask, onto a substrate with microhydrogel features functionalized with an anchoring agent.
2. Description of the Related Art
Electrospray Deposition (ESD) has lately emerged as an important technology for dispensing a small volume of a liquid composition by transforming it into a fine mist of droplets. Specifically, a high voltage electric field is applied to a capillary through which the liquid composition passes. In application, ESD may be used to spray or dispense various kinds of biomacromolecules and/or synthetic polymers, to form nano-sized particles and fibers, and to allow the biomacromolecules and/or synthetic polymers to accumulate and adhere on a substrate using electrostatic force. In addition, ESD has recently been widely used as an ionizer for mass spectrometers.
When a sample liquid stored in a thin capillary is supplied with several thousand volts relative to counter electrode, a strong electric field is generated at the tip of the capillary, due to the effect of electric field concentration. As the liquid begins to exit from the capillary, it forms a conical shape (Taylor cone) with the electrically charged ions gathered on its surface. Subsequently, when the electrostatic force becomes stronger than surface tension, the liquid erupts from the tip of the capillary to form a fine jet. Since the jet is highly charged, the liquid immediately turns into fine droplets to generate spray with each droplet split from the next by electrostatic force. Because the droplets formed by means of electrospray are tiny, the solvent evaporates and dries in a very short period of time. As a result, charged nano-particles may be formed, which may be subsequently attracted to the counter-electrode by electrostatic force. If desired, the liquid can be sprayed in a pattern controlled by masks made of insulating material and/or additional electrodes.
The particles of materials fabricated by ESD may have diameters of as little as less than dozens of nano-meters. Using this technique, various types of coating may be deposited on a surface, wherein the coatings may include materials like synthetic polymers and biomacromolecules (e.g. proteins). Thickness of the coating is controllable at the level of nano-meters, and with the electrostatic force, it is theoretically possible to coat the substrate both of flat surfaces or of complex shapes. By using a mask of insulating material in the ESD method, electrostatic force controls the pattern of deposition and forms it in desired shape, either spotted or striped. The resolution of the deposit ranges from micron-scale to sub-micron-scale.
In general, features of ESD may include, but are not limited to: (1) spraying and depositing various substances such as organic/inorganic compounds, biomacromolecules and/or synthetic polymers; (2) maintaining the sample relatively free from damage, as ESD processes may be conducted under desired temperature and atmospheric pressure; (3) creating nano-sized particles and fibers; (4) controlling the depositions and patterns of the particle using electrostatic force; and (5) sample deposition in larger areas with batch processes.
Lab-on-a-chip technologies have received considerable attention recently as a result of an integrated effort put forth by the life science and physical science community to create functional platforms for cell-surface interactions, early disease detection, personalized medicine, drug discovery, and single molecule analysis. By utilizing minimal volumes of the biological molecules and reagents, this technology may lead to higher throughput, improved sensitivity, and faster speed in scientific research and development.
To deposit samples on the chip, techniques such as spotting, ink-jet, and micro-contact printing have been developed and generally employed. ESD has also been used to generate arrays of protein in noncontact mode applicable to the creation of biochips, including that of protein chips. To that end, ESD may provide higher resolution and greater use in mass chip fabrication with batch processes, thereby overcoming technical limitations in conventional methods and meeting various laboratory needs.
While capillaries and electrically collimating dielectric masks have been used to make protein arrays using ESD processes, these techniques cannot be integrated in planar microfabrication schemes of the lab-on-a-chip and introduce an added complexity of aligning an ESD apparatus to a collimating mask, which needs to be further aligned to surface features formed on grounded protein array chips. Furthermore, to provide multiplexing capability to the ESD apparatus, it is important that multiple source tips of ESD apparatus be equidistant from the surface. A slight misalignment of the different source tips in ESD apparatus can cause the failure of all but one with the closest ground surface proximity, thereby limiting the multiplexing capability of the ESD apparatus.
Hence, there is a need for a robust and reliable ESD apparatus that can dispense and deposit, either simultaneously or sequentially, one or more samples in small volumes onto a substrate surface. Moreover, there is a need for an ESD apparatus that can be manufactured through convenient planar microfabrication and micromachining. Finally, there is a need for a reliable and efficient process to deposit samples on a substrate through a mask-less ESD process.